1. Field of the Invention
The present invention relates to high frequency power amplifiers and more particularly to techniques for combining, monolithically or otherwise, individual power amplifiers to achieve power combining and impedance transformation.
2. Description of the Related Art
The design of high frequency power amplifiers with reasonable power levels, efficiency and gain remains one of the major challenges in the pursuit of a single-chip integrated transceiver. Although several advances have been made in this direction, the design of a truly integrated power amplifier on a lossy substrate, such as silicon or silicon germanium has been an elusive goal.
Multiple external components such as bonding wires and external baluns have been used as tuned elements to produce output power levels in excess of 1W using CMOS transistors. See e.g., K. C. Tsai and P. R. Gray, xe2x80x9cA 1.9 GHz, 1-W CMOS Class-E Power Amplifier for Wireless Communications,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 34, no. 7, pp. 962-969, July 1999[1]; and C. Yoo and Q. Huang, xe2x80x9cA Common-Gate Switched, 0.9W Class-E Power Amplifier with 41% PAE in 0.25 xcexcm CMOS,xe2x80x9d Symposium on VLSI Circuits Digest, pp. 56-57, Honolulu, June 2000[2]. Similar performance levels have been achieved with Si-Bipolar transistors. See, e.g. W. Simbxc3xcrger, et al, xe2x80x9cA Monolithic Transformer Coupled 5-W Silicon Power Amplifier with 59% PAE at 0.9 GHz,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 34, no. 12, pp. 1881-1892, December 1999[3]; and W. Simbxc3xcrger, et al., xe2x80x9cA Monolithic 2.5V, 1W Silicon Bipolar Power Amplifier with 55% PAE at 1.9 GHz,xe2x80x9d IEEE MTT-S Digest, vol. 2, pp. 853-856, Boston, June, 2000[4].
Moreover, alternative technologies for active devices with higher breakdown voltages and higher substrate resistivity have been used to increase the output power and efficiency of integrated amplifiers. For example, LDMOS transistors with a breakdown voltage of 20V have been used on a semi-insulating substrate, but still this design delivers only 200 mW. See, Y. Tan, et al., xe2x80x9cA 900-MHz Fully Integrated SOI Power Amplifier for Single-Chip Wireless Transceiver Applications,xe2x80x9d IEEE Solid-State Circ., vol. 35, no. 10, pp. 1481-1485, October 2000[5]. Further, GaAs MESFET""s on insulating substrates have been used to integrate power amplifiers. J. Portilla, H. Garcia, and E. Artal, xe2x80x9cHigh Power-Added Efficiency MMIC Amplifier for 2.4 GHz Wireless Communications,xe2x80x9d IEEE Journal of Solid State Circuits, vol. 34, no. 1, pp. 120-123, January 1999[6]. Unfortunately, these technologies are significantly more costly and more difficult to manufacture than conventional silicon-based transistor technologies, such as CMOS.
A summary of these prior achievements in the design of high-frequency, low voltage power amplifiers is provided in Table 1:
Two significant problems in the design of a fully-integrated high speed solid state power amplifier using conventional silicon technologies such as CMOS are (1) the low resistivity of the lossy substrate which increases the loss of on-chip inductors and transformers; and (2) the low breakdown voltages of the transistors. These problems are exacerbated as the minimum feature sizes of the transistors (such as CMOS) are scaled down for faster operation.
More particularly, the high conductivity of lossy substrates causes long metal lines, including conventional spiral inductors fabricated on the same substrate, to be very lossy in terms of power. If the metal lines are made wide to reduce resistance, the capacitive coupling effect between the metal and substrate will drain part of the current to the substrate, thereby increasing power dissipation. On the other hand, if the metal lines are made narrow enough to effectively overcome this problem, the metal resistance will significantly increase, again absorbing (dissipating) a significant portion of the power.
The low breakdown voltage of conventional transistors such as CMOS, for example, limits the maximum allowable drain voltage swing of the transistor. This makes it necessary to perform some form of impedance transformation to achieve a larger output power. For example, a xc2x12V drain voltage swing delivers only 40 mW to a 50 xcexa9 load if no such impedance transformation is performed. While impedance transformation can be achieved using a 1:n transformer, unfortunately, an on-chip spiral 1:n transformer on a standard CMOS substrate is very lossy and will degrade the performance of the amplifier greatly. Alternatively, an on-chip resonant match could be used, but this technique also results in significant power loss.
In sum, as all high frequency power amplifiers ostensibly require some inductorsxe2x80x94essentially long metal linesxe2x80x94for matching purposes, connections for the supplies, and some form of power combining, conventional power amplifier circuits tend to be very power inefficient and not commercially viable above certain power and frequency levels.
Thus, it would be highly desirable to have a low cost, fully integratable topology for a power amplifier that can be fabricated with low cost, silicon-based processes and that can provide significant output power levels in the microwave and millimeter-wave frequency ranges. It would also be desirable if such a topology could be implemented with discrete power amplifiers as well as monolithic integration techniques. Ideally, this architecture would also be useful in the design of both lossy substrate IC""s as well as non-lossy substrate IC""s.
The present invention, which addresses these needs, resides in a distributed, circular-geometry, power amplifier as a means for power combining and impedance transformation to achieve a very high output power in a small package and to overcome the low breakdown voltage of conventional active devices such as short-channel MOS transistors.
In particular, the present invention resides in a distributed, circular geometry, power amplifier for amplifying an RF input signal that comprises a plurality of smaller push-pull amplifiers. Each amplifier includes two gain blocks that each has an input port with positive and negative terminals and an output port with positive and negative terminals. The two gain blocks of each push-pull amplifier are interconnected at the positive terminals of their respective output ports by an inductive path and share a common supply voltage to the positive terminals of their respective output ports. The negative terminal of the output port of each gain block of each push-pull amplifier is connected to negative terminal of the output port of a gain block of an adjacent push-pull amplifier such that the amplifiers are configured in an interconnected circular geometry, with the connected negative terminals of adjacent gain blocks being connected together to form a virtual ac ground.
In operation, the input port of each gain block is adapted to receive an ac input signal of at least substantially equal magnitude and opposite phase relative to the input port of an adjacent gain block. The push-pull amplifiers are interconnected such that, for the fundamental frequency of operation, virtual ac-grounds are presented at the negative terminals of the output ports of the gain blocks.
In a more detailed aspect of the present invention, the distributed circular geometry power amplifier comprises at least two push-pulls amplifiers designed to amplify an RF input signal. A first push-pull amplifier includes a first gain block and a second gain block, each block having an input port with positive and negative terminals and an output port with positive and negative terminals, the blocks being interconnected at the positive terminals of their respective output ports by an inductive path. A second push-pull amplifier includes a third gain block adjacent the second gain block and a fourth gain block, the third and fourth gain block each having an input port with positive and negative terminals and an output port with positive and negative terminals, the gain blocks of the second push-pull amplifier being interconnected at the positive terminals of their respective output ports by an inductive path. In order to create the xe2x80x9ccircularxe2x80x9d closed loop, the second and third gain blocks are interconnected at the negative terminals of their respective output ports and the negative terminal of the output port of the fourth gain block is connected to the negative terminal of the output port of the first gain block such that substantially all ac current that flows from the fourth gain block flows into the first gain block. The fourth and first gain blocks may, but typically will not, be directly connected to each other. In a typical configuration, at least (and preferable more than) one additional push-pull amplifier having a pair of interconnected gain blocks is provided between the fourth and first gain block, such that the negative terminal of the output port of the fourth gain block is indirectly connected to the negative terminal of the output port of the first gain block via this at least one additional push-pull amplifier.
In a more detailed embodiment, the power amplifier further includes a third and fourth push-pull amplifier, thereby creating a quad-push-pull power amplifier with eight gain blocks. In particular, the third push-pull amplifier has fifth and sixth gain blocks, each having an input port with positive and negative terminals and an output port with positive and negative terminals, the fifth and sixth blocks being interconnected at the positive terminals of their respective output ports by an inductive path. Similarly, the fourth push-pull amplifier has seventh and eighth gain blocks, each block having an input port with positive and negative terminals and an output port with positive and negative terminals, the seventh and eighth blocks being interconnected at the positive terminals of their respective output ports by an inductive path. The quad-amplifier device is interconnected such that the negative terminal of the output port of the fourth gain block is connected to the negative terminal of the output port of the fifth gain block, the negative terminal of output port of the sixth gain block is connected to the negative terminal of the output port of the seventh gain block, and the negative terminal of output port of the eighth gain block is connected to the negative terminal of the output port of the first gain block.
The gain blocks that comprise the push-pull amplifiers used by the present invention may take various configurations, depending on the desired gain, circuit complexity, cost and other factors. In one basic embodiment, each gain block of each push-pull amplifier comprises a single three-terminal active device, such as a CMOS or bipolar transistor, having a cathode, an anode, and a control terminal. In another embodiment, each gain block of each push-pull amplifier comprises a compound device having at least a first and a last three-terminal active device. The active devices of each gain block are connected together in a cascoded fashion such that the cathode of the first active device serves as the negative terminal of the output port of each gain block, the anode of the last active device serves as the positive terminal of the output port of each gain block, and the control terminal of the first active device is the input port of the gain block. With this configuration, each push-pull amplifier, and thus the power amplifier, can advantageously supply more gain than can a single transistor per gain block design.
The power amplifier of the present invention enables the push-pull amplifiers to be monolithically integrated onto a single chip. Moreover, the inductive path of each push-pull amplifier may simply be a metal slab and more particularly a substantially straight metal slab.
In yet further improvements to the design of the present invention, the power amplifier may further include a resonant, harmonic tuning capacitor that is connected between the positive terminals of the output ports of adjacent gain blocks of adjacent push-pull amplifiers. The amplifier may also include an inductive loop disposed between the input ports of adjacent gain blocks of adjacent push-pull amplifiers in order to tune the impedance presented to the RF input signal.
Turning to the RF input side, in order for the circuit to operate properly, a balanced input must be provided to all input ports of all gain blocks. To address this, an input power splitting network is included that symmetrically connects an in-phase balanced input signal to be amplified to the input ports of all gain blocks. The input power splitting network may symmetrically connect the in-phase balanced input signal from a point inside the circular geometry of the power amplifier or from points outside the circular geometry of the power amplifier.
In the preferred embodiment, the power amplifier further includes a power-combining circuit connected to the push-pull amplifiers that combines the signals amplified by each of the push-pull amplifiers. In order to achieve power-combining, the push-pull amplifiers are preferably configured as a first closed loop to form a circular geometry primary winding of an active transformer and the power-combining circuit is configured as a secondary winding of the active transformer that is located in proximity with and magnetically coupled to the primary winding. Thus, the secondary winding has a single output that provides the summed outputs of the push-pull amplifiers in the closed first loop. The secondary winding may be a single turn circuit or multiple turn circuit.
Furthermore, the secondary winding may advantageously comprise a single or multiple turn inductors formed by a variable width metal line. The metal line has sections that are relatively wide where a low ac voltage is present relative to the substrate and relatively narrow where a high ac voltage relative to the substrate is present. This geometry offers the advantage of further reducing the power loss, as it takes advantage of low metal resistance of wider metal where the ac voltage signal is low, thus reducing the loss and takes advantage of low capacitive coupling to the substrate of a narrower metal where the ac voltage is high, thus again reducing the loss. In this fashion, both the metal resistance loss and capacitive coupling loss are reduced
Turning momentarily back to the input circuit, the input power splitting network disclosed above may advantageously comprise a plurality of twisted input loops in proximity with the secondary winding, thereby providing magnetic coupling from the secondary winding. This geometry offers the advantage of further enhancing the gain or linearity of each push-pull amplifier in the power amplifier.
In yet a more detailed aspect of the preferred embodiment of the present invention, an additional secondary winding in proximity with and magnetically coupled to the primary and secondary windings may be provided to create an interdigitated transformer with its attendant benefits of lower power loss. Alternatively, or in addition to the multiple secondary winding improvement, the power amplifier of the present invention may further include at least one additional circular geometry primary winding in proximity with and magnetically coupled to the primary and secondary windings to create an interdigitated transformer.
A method of combining the amplified outputs of a plurality of push-pull amplifiers to form a power amplifier is also disclosed. In this method, each amplifier includes two inductively-gain blocks interconnected by an inductive path. The method comprises configuring the plurality of amplifiers to form a first closed loop such that adjacent gain blocks of adjacent amplifiers are interconnected and as so interconnected, form virtual ac grounds at their junctions, and driving adjacent gain blocks of adjacent push-pull amplifiers with at least substantially equal and opposite input signals. In a more detailed aspect of the present invention, the method further includes combining the output power of the push-pull amplifiers in the first closed loop in a secondary coil that is located in proximity with and magnetically coupled to the first closed loop.
A low loss inductor for deposition on a substrate of an integrated circuit that processes voltage signals is also described. The inductor includes an elongated conductive body deposed on the substrate and having first and second ends, conductive sections disposed between the ends, and an average ac signal voltage acorss the body, such that a section where the signal voltage is determined to be lower than the average ac signal voltage across the body is relatively wider than another section of the inductor whereat the signal voltage is determined to be higher than the average ac signal voltage across the body.
Additionally described is a method for reducing the electrical losses of an inductor deposed on a substrate of an integrated circuit, the inductor have an elongated body with interconnected conducting sections, an average width and an average ac signal voltage across the body. The method includes decreasing the width of a section of the body of the inductor relative to the average width whereat the ac voltage signal on the section is relatively higher than the average ac signal voltage across the inductor body; and increasing the width of another section of the body of the inductor whereat the ac voltage signal on the other section is relatively lower than the average ac signal voltage across the inductor body.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.